Charlotte Renfield-Miller, left, and Claire Breedlove in Harlem, where the friends recently split the purchase of a house with separate units. Lisa Corson for The New York Times

Claire Breedlove was happy in her Harlem neighborhood, though not in her rental, which was in a poorly maintained building. Last winter, she decided to buy a place of her own, but was immediately disappointed when she was outbid on a nearby one-bedroom condominium with a terrace.